Interpretation of Time Spectra Measured in High-Intensity Shear Flows

Fisher and Davies have suggested several mechanisms which can prevent the direct interpretation of time spectra as space spectra by the frozen pattern hypothesis. It is shown that, when various criteria such as that of Lin are satisfied, all but one of these mechanisms, the fluctuation of convection velocity, can be disregarded at high frequencies. An analytical model of this mechanism is constructed and several of its properties examined. On the basis of this model, a correction formula is obtained for the high-frequency part of the one-dimensional streamwise spectrum. This is applied to experimental data taken in a curved turbulent mixing layer. It is found that even under extreme circumstances corrections from this source are not likely to exceed 30%.